This invention relates to a color picture tube which is constructed by disposing close to a phosphor screen a plurality of spaced shadow masks provided with a large number of apertures allowing for the passage of electron beams, and more particularly to a shadow mask assembly.
One of color picture tubes provided with a plurality of shadow masks known to date is a mask focusing type. Electron beams are very efficiently utilized in such mask focusing type color picture tube, in which a plurality of spaced shadow masks are respectively provided with prescribed different potentials; and an electron lens for focusing electron beams passing through the apertures of the shadow masks is formed between the shadow masks. Such mask focusing type color picture tube is already set forth in, for example, the U.S. Pat. Nos. 2,971,117 and 3,398,309, Japanese patent publication 38-22,030 and Japanese utility model publication 47-20,451.
Other examples of a color picture tube provided with two parallel shadow masks are also disclosed in the Japanese patent publication 55-2698, Japanese patent application disclosure 50-57575 and Japanese utility model application disclosure 48-93,769. The ordinary color picture tube provided with a single shadow mask has the drawbacks that the impingement of electron beams on the shadow mask results in its thermal deformation, preventing electron beams from being irradiated on the prescribed phosphor regions on a phosphor screen, that is, giving rise to the so-called "mislanding" and consequently a decline in color purity. To avoid such drawbacks, the color picture tubes proposed in the aforementioned publication are characterized in that two parallel shadow masks are provided, thereby shutting off electron beams not required to impinge on the phosphor screen in order to suppress as much as possible the temperature rise of the second mask which plays an important role in prescribing the color purity.
With the above-mentioned focusing type color picture tube and color picture tube particularly designed to prevent the so-called mislanding, the apertures formed in one of the two shadow masks should occupy the corresponding positions to those of the other shadow mask. However, considerable difficulties are encountered in manufacturing such shadow mask assembly. According to the process of producing a mask assembly which is disclosed in, for example, the Japanese patent publications 47-8261 and 47-28,188, an insulating material such as glass is filled between the two shadow masks except for the apertures in order to effect insulation between said two shadow masks, presenting difficulties in manufacturing a mask assembly by press molding. Further, the insulating material tends to be charged by the impingement of electron beams thereon, thus harmfully affecting electron beams passing through the apertures. Therefore, any of the proposed shadow mask assembly-manufacturing methods fails to be put to practical application.
In contrast, the Japanese patent application disclosure 57-138,746 sets forth a practicable shadow mask assembly-manufacturing method which comprises the steps of superposing a plurality of flat metal plates and press-molding them together without interposing an insulating material therebetween, thus assuring the accurate alignment of the mutually facing apertures of the superposed flat metal plates. However, the shadow mask assembly-manufacturing method set forth in the Japanese patent disclosure 57-138,746 has the drawback that when the peripheral or so-called skirt sections of the assembled shadow masks are bent by press work, said skirt sections tend to touch each other, even though the mutually facing planes of the greater part of the assembled shadow masks are spaced from each other at a prescribed distance during fitting to the color picture tube, making it impossible to impress different potentials on the respective shadow masks. Further even when it is designed to impress the same potential on the two shadow masks, the immediate conduction of heat from the first shadow mask to the second shadow mask makes it impossible to suppress the temperature rise of said shadow mask, namely, its resultant thermal deformation. For resolution of the above-mentioned difficulty, it may be contemplated to interpose an insulating material between the skirt sections of the assembled shadow masks. When, however, a shadow mask assembly is constructed by superposing a plurality of flat metal plates by press forming, there is no room between the skirt sections of the superposed shadow masks to allow for the insertion of an insulating material. If it is tried to provide a space of the insertion of an insulating material between said skirt sections, it is necessary to reduce the thickness of the skirt sections of the shadow masks. However, this process objectionably decreases the mechanical strength of the shadow masks. Moreover, the shadow mask metal and insulating material have different degree of ductility. Therefore, extreme difficulties arise in effecting the press molding of two superposed shadow masks with an insulating material interposed therebetween. Further when the color picture tube is heated at 400 to 500.degree. C. during the manufacture of the color picture tube with an insulating material interposed between the shadow masks, then various difficulties arise due to the different physical properties, for example, thermal expansion coefficients of the shadow mask metal sheets and the intervening insulating material, and also due to the different chemical properties, for example, releasabilities of the impurity gases contained in the shadow mask metal sheets and the intervening insulating material. Therefore, the last mentioned known color picture tube-manufacturing method fails to be put to practical application.